1. Field of the Invention
This invention is directed to the vapor-phase disproportionation of toluene with a very low hydrogen to hydrocarbon mole ratio and in the presence of a crystalline aluminosilicate zeolite characterized by a silica/alumina mole ratio of at least 12, a constraint index within the approximate range of 1 to 12 and exceptionally long catalyst life, being capable of affording high yield of disproportionation product, a favorable selectivity toward para-xylene, and which is easily and effectively regenerated, when necessary, without substantial loss in activity.
2. Discussion of the Prior Art
U.S. Pat. No. 3,551,509 discloses transalkylation between trimethylbenzenes and toluene to yield xylenes and benzene in the presence of a crystalline aluminosilicate catalyst having pore openings of 8 to 15 Angstrom units and, preferably containing Group VIII metals, hydrogen and rare earth cations. The above patent is also the subject of U.S. Pat. No. Re. 27,639. From the teaching of these patents, one would expect that the rather large pore openings of 8 to 15 Angstrom units is a requirement for effective transalkylation of polyalkylaromatic hydrocarbons with aromatic hydrocarbons such as toluene.
In the area of aromatic disproportionation, Grandio et al. teach in the Oil and Gas Journal, Vol. 69, Number 48(1971) a liquid-phase toluene disproportionation process utilizing zeolite catalysts in the absence of hydrogen. They further teach that vapor-phase toluene disproportionation requires hydrogen recycle or else frequent regeneration of catalyst to keep coke levels low on the catalyst and to maintain catalytic activity over any reasonable period of time.
Otani teaches in Chemical Engineering, 77(16), 118(1970) that vapor-phase catalytic disproportionation of toluene requires hydrogen recycle to keep the zeolite catalyst from excessive coke build-up and, thereby, maintain reasonable catalyst activity.
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879; and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
Unfortunately, while the crystalline aluminosilicate catalysts proposed for such prior art methods provide satisfactory initial yields of desired products of disproportionation, for the most part, their catalytic aging properties are not sufficiently good enough to warrant commercial application, even with high hydrogen to hydrocarbon ratios. Hence, it is of advantage to provide a satisfactory process for toluene disproportionation using a crystalline aluminosilicate zeolite catalyst which has improved aging properties, i.e. maintains disproportionation in high yield over a long, commercially attractive period of time, heretofore lacking in the art. Added advantage for the present process is that very little or no hydrogen is required and that the para-xylene content of the xylenes produced is higher than equilibrium predictions.